1. Field of the Invention
The present invention relates to a display device including a display section having a photo emission element and a pixel circuit for each pixel, and a method of driving the display device, and to an electronic device having such a display device.
2. Description of the Related Art
Recently, in a field of a display device performing image display, a display device is developed and progressively commercialized, which uses a current-drive optical element, for example, an organic EL (Electro Luminescence) element, as a photo emission element of a pixel, emission luminance of the optical element varying depending on a current value.
The organic EL element is a self-luminous element unlike a liquid crystal element. Therefore, a display device using the organic EL element (organic EL display device) does not need a light source (backlight), and therefore the display device is high in visibility of an image, low in power consumption, and high in response speed of an element compared with a liquid crystal display device needing a light source.
In the organic EL display device, a drive method includes a simple (passive) matrix method and an active matrix method as in the liquid crystal display device. The former has a simple structure, but has a difficulty that a large, high-resolution display device is hardly achieved. Therefore, the active matrix method is currently actively developed. In this method, a current flowing into a photo emission element disposed for each pixel is controlled by an active element (typically, TFT (Thin Film Transistor) provided in a drive circuit provided for each photo emission element.
Generally, a current-voltage (I-V) characteristic of the organic EL element is deteriorated with time (aged deterioration). In a pixel circuit for current drive of the organic EL element, when the I-V characteristic of the organic EL element is changed with time, a voltage-dividing ratio of the organic EL element to a drive transistor connected in series to the EL element is changed, and therefore a voltage Vgs between a gate and source of the drive transistor is also changed. As a result, since a value of current flowing into the drive transistor is also changed, a value of current flowing into the organic EL element is also changed, and consequently emission luminance is changed in accordance with the current value.
In some cases, a threshold voltage Vth or mobility μ of the drive transistor is temporally changed, or the threshold voltage Vth or mobility μ varies for each pixel circuit due to variation in manufacturing process. When the threshold voltage Vth or mobility μ of the drive transistor varies for each pixel circuit in this way, a value of current flowing into the drive transistor varies for each pixel circuit. Therefore, even if the same voltage is applied to a gate of the drive transistor, emission luminance of the organic EL element may vary, leading to loss in uniformity of a screen.
Thus, a proposal has been made in order to achieve that even if the I-V characteristic of the organic EL element is changed with time, or even if the threshold voltage Vth or mobility μ of the drive transistor is changed with time, emission luminance of the organic EL element is kept to a certain luminance without being affected by such change. Specifically, a display device is developed, which incorporates a function of compensating variation in I-V characteristic of the organic EL element, and a function of correcting variation in threshold voltage Vth or in mobility μ of the drive transistor (for example, described in Japanese Unexamined Patent Application Publication No. 2008-33193).